1. Field of the Invention
The present invention relates to an optical signal-treating apparatus that utilizes a nonlinear optical phenomenon in optical fiber.
2. Description of the Background Art
When a nonlinear optical phenomenon is produced by inputting high power light into an optical fiber, the optical fiber can generate not only broad-band light (supercontinuum light) but also light having a wavelength different from that of the inputted light. As the effective area of the optical fiber is decreased, and as the nonlinear coefficient in the vicinity of the core is increased, the nonlinear optical phenomenon is intensified. To decrease the effective area, it is necessary to decrease the core diameter in general. On the other hand, to increase the nonlinear coefficient, it is necessary to perform a treatment such as the increasing of the concentration of the germanium oxide (GeO2) and the like to be doped in the core.
An optical fiber having a high nonlinear can produce a nonlinear optical phenomenon with high efficiency. However, this type of fiber has a small core diameter, in general. Therefore, it is difficult to couple the light outputted from the pulse light source to the optical fiber with stability and high efficiency. In particular, when the laser light source produces a spatial output as with a solid laser, if the relative position between the laser light source and the optical fiber varies unstably, then the power of the light traveling over the optical fiber will fluctuate.
It is known that when the power or pulse width of the pulse light to be inputted into the optical fiber varies, the spectral line shape of the broad-band light generated in the optical fiber varies (for example, see a literature written by W. J. Wadsworth et al.: J. Opt. Soc. Am. B/Vol. 19 (2002) p. 2148 and another literature written by A. B. Fedotov et al.: J. Opt. Soc. Am. B/Vol. 19 (2002) p. 2156). When a desired output property is intended to obtain stably from an optical fiber utilizing a nonlinear optical phenomenon, it is necessary to perform a control so that the power and pulse width of the pulse light to be inputted into the optical fiber can be stabilized.
Engineers have been studying the coupling between a laser light source and an optical fiber from various points of view. In particular, a large number of reports have been published on the coupling between a semiconductor laser light source and an optical fiber. Most of these reports study how to couple the high power light outputted from a laser light source to an optical fiber with low power loss. There are no reports that specify the peak power, pulse shape, and spectral line shape of the pulse light to be inputted into a nonlinear optical fiber. In other words, no control has been performed on the condition for inputting the pulse light into a highly nonlinear optical fiber, although the foregoing condition is important to obtain broad-band light and wavelength-converted light both having a desired property.